Parry arc (thing)

On April 8, 1820, the celebrated English explorer William Edward Parry was icebound with his crew in the ships Griper and Hecla off of Melville Island in the Canadian Arctic. They had been fixed in place for seven months while trying to find a northwest passage between Greenland and the Bering Strait. They were not guaranteed to escape, and in fact were not to break free for another three months. Despite the dire conditions, or perhaps because of them (giving in to boredom risks cabin fever), Parry took the time to accurately document an amazing complex halo display that he saw in the sky that day.

Additionally, his notes showed a strange new, colorful arc that seemed to cap the upper tangent arc. It was not known to have been documented before. This was probably because the conditions that lead to this arc are very rare: singly-oriented needle-shaped ice crystals, floating with their long axes and two opposing side faces horizontal. Just a few crystals in this orientation are enough to create the new arc he saw, now called the Parry arc. The particular orientation of crystals is called the Parry orientation. When a large number of crystals orient this way, their internal ray paths create the Parry arcs in addition to other halos visible at the time.

Like the tangent arcs, Parry arcs change shape with rising elevations of the sun. Both types of halos are formed by sunlight passing into an edge face of the crystal and diffracting out the next adjacent face plus one, making a 60° bend from sun to eye.

Two upper Parry arcs
At the horizon, only one Parry arc is visible, hugging the upper tangent arc. At 5° it lifts a few degrees above the upper tangent arc but maintains its suncave "V" shape. A second upper Parry arc, a 30° long sunvex and gently curving arc, begins to appear just above. At 10° the suncave "V" rises more and touches the stronger suncave arc. At 15° the sunvex arc disappears altogether. The long suncave arc slowly strengthens, and flattens, and moves closer to the 22° halo as the sun continues to rise. At its strongest, the suncave arc sweeps across almost 30° in the sky.

A shy lower Parry arc
The lower Parry arc is much more shy, appearing after the sun is higher than 25°. At this elevation it hugs the lower tangent arc tightly. At 30° it lowers a few degrees, but does not significantly change shape. In simulation, two more flat Parry arcs appear near the bottom of the 22° halo when the sun is at 40° elevation and join together to form a huge bowl that holds the sun. Along with the long suncave upper Parry arc and the 22° halo, this display looks like a giant human eye. Unfortunately this eye in the sky has yet to be documented or photographed.